The present invention generally relates to very low frequency field, and especially relates to a phase shifter and power amplifier and magnetic resonance imaging apparatus in the nuclear magnetic resonance imaging technology.
The nuclear magnetic resonance imaging technology is applied broadly in the medical field at present. The magnetic resonance imaging (MRI) is also called nuclear magnetic resonance imaging (NMRI), which is a diagnostic method that generates an image by atomic nucleus resonance in the magnetic field. The basic principle is forming an image by utilizing the inherent characteristics of atomic nucleus and the interaction of a magnetic field. A type of atomic nucleus closely related to tissues of the human body generates magnetic resonance signals under the effect of an external radio frequency field. A set of parameters related to the magnetic resonance may serve as imaging variables. A power amplifier is one of the components within a magnetic resonance imaging apparatus.
As is known, power amplifier linearity technology, for example, feedback, feed forward and predistortion, etc., is the mainstream in enhancing the Adjacent Channel Power Ratio (ACPR) and improving the power amplifier linearity. This technology compares the amplitude and phase of the input signal and the output signal, thus, the phase shift function of a phase shifter is employed.
A known current phase shifter is shown in FIG. 1. In the current phase shifter, capacitance C1 and C2 are connected in series between an input port Pin and an output port Pout. A diode 8 and a diode 9 connected in parallel are grounded at one end and are connected to a capacitance C3 at the other end. The other end of the capacitance C3 is connected to an inductance L. The other end of the inductance L is connected between the capacitance C1 and C2. A control voltage is applied between the capacitance C3 and the diodes 8, 9 connected in parallel via a resistance R1. For example, the input port Pin and the output port Pout are ports with 50 Ohm, where the working frequency is about 64 Mhz, the capacitances C1, C2 and C3 are approximately 3.3 nF, the inductance L is about 47 nH, and the resistance R1 is about 1 k Ohm. Other values may be possible. As shown in FIG. 1, the impedance of the diode 8 and diode 9 will vary with the changes of the bias voltage. The changes of the impedances of the diode 8 and diode 9 inevitably cause phase variation from the output port Pout to the input port Pin, thereby achieving the purpose of phase shift. However, the changes of the phase shift caused by this example is about 26 degrees, thus the range of phase shift is too small, while the insertion loss is rather large, which influences system gain and noise.
Another known current phase shift is shown in FIG. 2. An input port Pin is connected to an input end of a bridge 3, and an output port Pout is connected to an isolation port of the bridge 3. The anodes of two varactors 4 and 5 are respectively connected to two power dividing ports of the bridge 3. The two power dividing ports are also respectively connected to grounding resistance R2 and R5. The cathodes of the two varactors 4 and 5 are respectively connected to resistance R3 and R4 and are connected in series with a control voltage. A capacitance C4 is connected to control voltage at one end, and is grounded at the other end. Resistances R2, R3, R4, R5 provide current protection and voltage bias for the two varactors, and the capacitance C4 performs the function of blocking and filtering. This type of phase shift technology is very popular and may be applied to many fields, especially in mobile communication and radar fields. In this example, it is assumed that the input port Pin and the output port Pout are ports with 50 Ohm. It can be seen from FIG. 2 that the reactances of the varactor 4 and varactor 5 vary with the variation of the control voltage, so the phase variation of the emitted signals will be different, and thus a phase delay is realized.
Compared to the phase shifter shown in FIG. 1, the phase shifter shown in FIG. 2 shifts phase in a broader range, however, it is difficult to realize a phase shift of 180 degrees.